Pedestrian warning systems for hybrid and electric vehicles and other vehicles that run more quietly than those with internal combustion engines typically generate a predetermined warning signal and play it from one or more external speakers. The warning sounds should only be as loud as necessary to serve as a pedestrian warning, and heard only by people outside and in the path of the vehicle. Some such systems adjust the warning sound output level based on the background ambient noise level. However, a human's perception of sound in the presence of noise depends on the frequency and level of the noise. Accordingly, the perceived volume of pedestrian warning sounds may be insufficient in certain circumstances or may be too loud in other circumstances.